Various processes have been heretofore tried for the purpose of obtaining ethylene/.alpha.-olefin/diene copolymer rubbers having good processability (high flowability).
For example, Japanese Patent Publication No. 46(1971)-29012 describes that a specific ethylene/propylene rubber composition composed of 20-80 % by weight of an ethylene/propylene rubber having inherent viscosity of not more than 1.0 and 80-20 % by weight of an ethylene/propylene rubber having inherent viscosity of not less than 2.5 is excellent in the processability. In this publication, however, any ethylene/.alpha.-olefin/diene rubber improved in other properties such as fatigue resistance and mechanical strength is not disclosed.
Japanese Patent Laid-open Publication No. 49(1974)-8541 discloses a process of mixing 15-65 % by weight of a high-molecular ethylene/propylene rubber having an average molecular weight of 40,000 to 200,000 and 35-85 % by weight of a low-molecular ethylene/propylene rubber having an average molecular weight of 2,000 to 30,000 with each other to improve the flowability of the resulting rubber, and also describes that the amount of the low-molecular ethylene/propylene rubber used in this process preferably is as large as possible to improve the flowability. Either in the publication, however, any ethylene/.alpha.-olefin/diene rubber improved in other properties such as fatigue resistance and mechanical strength is not disclosed.
Further, Japanese Patent Publication No. 59(1984)-14497 discloses a specific ethylene copolymer rubber composition as an ethylene copolymer rubber composition having high processability and high physical properties in the vulcanization process, which comprises:
5-70 parts by weight of an ethylene/.alpha.-olefin/polyene copolymer rubber (A) having an intrinsic viscosity [.eta.].sub.A of 0.8 to 6.0 dl/g as measured in decalin of 135.degree. C., an iodine value of 8 to 50 and a molar ratio of ethylene/.alpha.-olefin within the range of 50/50 to 95/5; and
30-95 parts by weight of an ethylene/.alpha.-olefin/polyene copolymer rubber (B) having an intrinsic viscosity [.eta.].sub.B of 0.1 to 5.0 dl/g as measured in decalin of 135.degree. C., satisfying the condition of [.eta.].sub.A /[.eta.].sub.B .gtoreq.1.2, and having an iodine value of smaller than the iodine value of the rubber (A) by 3 to 45 and within the range of 3 to 40 and a molar ratio of ethylene/.alpha.-olefin within the range of 50/50 to 95/5.
Furthermore, Japanese Patent Laid-open Publication No. 53(1988)-22551 describes an improved rubber composition employable for rubber vibration insulator. Generally, a high-molecular EPDM is good in the resistance to dynamic fatigue but poor in the processability. The above publication describes that the processability can be enhanced by blending a high-molecular EPDM with a low-molecular EPDM, and also describes that the low-molecular EPDM in the unvulcanized state serves as an processing aid and participates in the crosslinking reaction after the vulcanization to greatly contribute to enhancement in the dynamic fatigue resistance. However, the present inventors have found that when the low-molecular component is more or less incorporated by the crosslinking reaction, the obtained vulcanized rubber molded product is deteriorated not only in the compression permanent strain properties but also in the fatigue resistance.
By the way, physical properties required for EPDM used for manufacturing sponge rubber articles such as window frames of automobiles include shape retention properties indicating whether the same shape as that of a nozzle can be retained or not until the vulcanization of the EPDM is complete, processability (flowability), and surface smoothness of the vulcanized rubber molded product obtained by the vulcanization molding process.
In many cases, a static force is applied to the vulcanized rubber articles, so that EPDM of high molecular weight (high Mooney viscosity) is demanded as a material which is not physically deformed for a long period of time.
However, it is difficult to prepare a rubber satisfying all of processability (flowability) of EPDM, surface smoothness of the vulcanized rubber article obtained therefrom, shape retention properties in the vulcanization molding process and low-compression permanent strain properties at the same time.
Accordingly, eagerly demanded is an ethylene/.alpha.-olefin/diene rubber composition which is excellent in the processability (flowability) and the shape retention properties in the vulcanization molding process and capable of providing a vulcanized rubber molded product enhanced in surface smoothness, mechanical strength and resistance to dynamic fatigue.
The present invention intends to solve the above-mentioned problems accompanied by the prior art. The object of the present invention is to provide an ethylene/.alpha.-olefin/diene rubber composition which is excellent in processability (flowability) and shape retention properties in the vulcanization molding process and capable of giving a vulcanized rubber molded product enhanced in surface smoothness, mechanical strength and resistance to dynamic fatigue. Further, the object of the invention is to provide a vulcanized rubber molded product prepared by vulcanizing the above-mentioned rubber composition.